Storage system spanning multiple failure domains

ABSTRACT

A plurality of failure domains are communicatively coupled to each other via a network, and each of the plurality of failure domains is coupled to one or more storage devices. A failure resilient stripe is distributed across the plurality of storage devices, such that two or more blocks of the failure resilient stripe are located in each failure domain.

PRIORITY CLAIM

The present application is a continuation of U.S. Ser. No. 16/275,737filed Feb. 14, 2019, which claims priority to the following application,which is hereby incorporated herein by reference:

U.S. provisional patent application 62/683,841 titled “STORAGE SYSTEMSPANNING MULTIPLE FAILURE DOMAINS” filed on Jun. 12, 2018.

BACKGROUND

Limitations and disadvantages of conventional approaches to data storagewill become apparent to one of skill in the art, through comparison ofsuch approaches with some aspects of the present method and system setforth in the remainder of this disclosure with reference to thedrawings.

INCORPORATION BY REFERENCE

U.S. patent application Ser. No. 15/243,519 titled “Distributed ErasureCoded Virtual Filesystem” is hereby incorporated herein by reference inits entirety.

BRIEF SUMMARY

Methods and systems are provided for building a storage system spanningmultiple failure domains in a distributed filesystem substantially asillustrated by and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates various example configurations of a distributedfilesystem in accordance with aspects of this disclosure.

FIG. 2 illustrates an example configuration of a distributed filesystemnode in accordance with aspects of this disclosure.

FIG. 3 illustrates another representation of a distributed filesystem inaccordance with an example implementation of this disclosure.

FIG. 4 illustrates another representation of a distributed filesystem inaccordance with an example implementation of this disclosure.

FIG. 5 is a flowchart illustrating an example method for generating adistributed filesystem in accordance with an example implementation ofthis disclosure.

DETAILED DESCRIPTION

Traditionally, filesystems use a centralized control over the metadatastructure (e.g., directories, files, attributes, file contents). If alocal filesystem is accessible from a single server and that serverfails, the filesystem's data may be lost if as there is no furtherprotection. To add protection, some filesystems (e.g., as provided byNetApp) have used one or more pairs of controllers in an active-passivemanner to replicate the metadata across two or more computers. Othersolutions have used multiple metadata servers in a clustered way (e.g.,as provided by IBM GPFS, Dell EMC Isilon, Lustre, etc.). However,because the number of metadata servers in a traditional clustered systemis limited to small numbers, such systems are unable to scale.

The systems in this disclosure are applicable to small clusters and canalso scale to many, many thousands of nodes. An example embodiment isdiscussed regarding non-volatile memory (NVM), for example, flash memorythat comes in the form of a solid-state drive (SSD). The NVM may bedivided into 4 kB blocks and 128 MB chunks. Extents may be stored involatile memory, e.g., RAM for fast access, backed up by NVM storage aswell. An extent may store pointers for blocks, e.g., 256 pointers to 1MB of data stored in blocks. In other embodiments, larger or smallermemory divisions may also be used. Metadata functionality in thisdisclosure may be effectively spread across many servers. For example,in cases of “hot spots” where a large load is targeted at a specificportion of the filesystem's namespace, this load can be distributedacross a plurality of nodes.

FIG. 1 illustrates various example configurations of a distributedfilesystem in accordance with aspects of this disclosure. Shown in FIG.1 is a local area network (LAN) 102 comprising one or more nodes 120(indexed by integers from 1 to J, for j≥1), and optionally comprising(indicated by dashed lines): one or more dedicated storage nodes 106(indexed by integers from 1 to M, for M≥1), one or more compute nodes104 (indexed by integers from 1 to N, for N≥1), and/or an edge routerthat connects the LAN 102 to a remote network 118. The remote network118 optionally comprises one or more storage services 114 (indexed byintegers from 1 to K, for K≥1), and/or one or more dedicated storagenodes 115 (indexed by integers from 1 to L, for L≥1).

Each node 120 _(j) (j an integer, where 1≤j≤J) is a networked computingdevice (e.g., a server, personal computer, or the like) that comprisescircuitry for running processes (e.g., client processes) either directlyon an operating system of the device 104 n and/or in one or more virtualmachines running in the device 104 _(n).

The compute nodes 104 are networked devices that may run a virtualfrontend without a virtual backend. A compute node 104 may run a virtualfrontend by taking a single root input/output virtualization (SR-IOV)into the network interface card (NIC) and consuming a complete processorcore. Alternatively, the compute node 104 may run the virtual frontendby routing the networking through a Linux kernel networking stack andusing kernel process scheduling, thus not having the requirement of afull core. This is useful if a user does not want to allocate a completecore for the filesystem or if the networking hardware is incompatiblewith the filesystem requirements.

FIG. 2 illustrates an example configuration of a node in accordance withaspects of this disclosure. A node comprises a frontend 202 and driver208, a memory controller 204, a backend 206, and an SSD agent 214. Thefrontend 202 may be a virtual frontend; the memory controller 204 may bea virtual memory controller; the backend 206 may be a virtual backend;and the driver 208 may be a virtual drivers. As used in this disclosure,a virtual filesystem (VFS) process is a process that implements one ormore of: the frontend 202, the memory controller 204, the backend 206,and the SSD agent 214. Thus, in an example implementation, resources(e.g., processing and memory resources) of the node may be shared amongclient processes and VFS processes. The processes of the VFS may beconfigured to demand relatively small amounts of the resources tominimize the impact on the performance of the client applications. Thefrontend 202, the memory controller 204, and/or the backend 206 and/orthe SSD agent 214 may run on a processor of the host 201 or on aprocessor of the network adaptor 218. For a multi-core processor,different VFS process may run on different cores, and may run adifferent subset of the services. From the perspective of the clientprocess(es) 212, the interface with the virtual filesystem isindependent of the particular physical machine(s) on which the VFSprocess(es) are running. Client processes only require driver 208 andfrontend 202 to be present in order to serve them.

The node may be implemented as a single tenant server (e.g., bare-metal)running directly on an operating system or as a virtual machine (VM)and/or container (e.g., a Linux container (LXC)) within a bare-metalserver. The VFS may run within an LXC container as a VM environment.Thus, inside the VM, the only thing that may run is the LXC containercomprising the VFS. In a classic bare-metal environment, there areuser-space applications and the VFS runs in an LXC container. If theserver is running other containerized applications, the VFS may runinside an LXC container that is outside the management scope of thecontainer deployment environment (e.g. Docker).

The node may be serviced by an operating system and/or a virtual machinemonitor (VMM) (e.g., a hypervisor). The VMM may be used to create andrun the node on a host 201. Multiple cores may reside inside the singleLXC container running the VFS, and the VFS may run on a single host 201using a single Linux kernel. Therefore, a single host 201 may comprisemultiple frontends 202, multiple memory controllers 204, multiplebackends 206, and/or one or more drivers 208. A driver 208 may run inkernel space outside the scope of the LXC container.

A SR-IOV PCIe virtual function may be used to run the networking stack210 in user space 222. SR-IOV allows the isolation of PCI Express, suchthat a single physical PCI Express can be shared on a virtualenvironment and different virtual functions may be offered to differentvirtual components on a single physical server machine. The I/O stack210 enables the VFS node to bypasses the standard TCP/IP stack 220 andcommunicate directly with the network adapter 218. A Portable OperatingSystem Interface for uniX (POSIX) VFS functionality may be providedthrough lockless queues to the VFS driver 208. SR-IOV or full PCIephysical function address may also be used to run non-volatile memoryexpress (NVMe) driver 214 in user space 222, thus bypassing the Linux IOstack completely. NVMe may be used to access non-volatile storage media216 attached via a PCI Express (PCIe) bus. The non-volatile storagemedia 220 may be, for example, flash memory that comes in the form of asolid-state drive (SSD) or Storage Class Memory (SCM) that may come inthe form of an SSD or a memory module (DIMM). Other example may includestorage class memory technologies such as 3D-XPoint.

The SSD may be implemented as a networked device by coupling thephysical SSD 216 with the SSD agent 214 and networking 210.Alternatively, the SSD may be implemented as a network-attached NVMe SSD222 or 224 by using a network protocol such as NVMe-oF (NVMe overFabrics). NVMe-oF may allow access to the NVMe device using redundantnetwork links, thereby providing a higher level or resiliency. Networkadapters 226, 228, 230 and 232 may comprise hardware acceleration forconnection to the NVMe SSD 222 and 224 to transform them into networkedNVMe-oF devices without the use of a server. The NVMe SSDs 222 and 224may each comprise two physical ports, and all the data may be accessedthrough either of these ports.

Each client process/application 212 may run directly on an operatingsystem or may run in a virtual machine and/or container serviced by theoperating system and/or hypervisor. A client process 212 may read datafrom storage and/or write data to storage in the course of performingits primary function. The primary function of a client process 212,however, is not storage-related (i.e., the process is only concernedthat its data is reliably stored and is retrievable when needed, and notconcerned with where, when, or how the data is stored). Exampleapplications which give rise to such processes include: email servers,web servers, office productivity applications, customer relationshipmanagement (CRM), animated video rendering, genomics calculation, chipdesign, software builds, and enterprise resource planning (ERP).

A client application 212 may make a system call to the kernel 224 whichcommunicates with the VFS driver 208. The VFS driver 208 puts acorresponding request on a queue of the VFS frontend 202. If several VFSfrontends exist, the driver may load balance accesses to the differentfrontends, making sure a single file/directory is always accessed viathe same frontend. This may be done by sharding the frontend based onthe ID of the file or directory. The VFS frontend 202 provides aninterface for routing filesystem requests to an appropriate VFS backendbased on the bucket that is responsible for that operation. Theappropriate VFS backend may be on the same host or it may be on anotherhost.

A VFS backend 206 hosts several buckets, each one of them services thefilesystem requests that it receives and carries out tasks to otherwisemanage the virtual filesystem (e.g., load balancing, journaling,maintaining metadata, caching, moving of data between tiers, removingstale data, correcting corrupted data, etc.)

A VFS SSD agent 214 handles interactions with a respective storagedevice 216. This may include, for example, translating addresses, andgenerating the commands that are issued to the storage device (e.g., ona SATA, SAS, PCIe, or other suitable bus). Thus, the VFS SSD agent 214operates as an intermediary between a storage device 216 and the VFSbackend 206 of the virtual filesystem. The SSD agent 214 could alsocommunicate with a standard network storage device supporting a standardprotocol such as NVMe-oF (NVMe over Fabrics).

FIG. 3 illustrates another representation of a distributed filesystem inaccordance with an example implementation of this disclosure. In FIG. 3,the element 302 represents memory resources (e.g., DRAM and/or othershort-term memory) and processing (e.g., x86 processor(s), ARMprocessor(s), NICs, ASICs, FPGAs, and/or the like) resources of variousnode(s) (compute, storage, and/or VFS) on which resides a virtualfilesystem, such as described regarding FIG. 2 above. The element 308represents the one or more physical storage devices 216 which providethe long term storage of the virtual filesystem.

As shown in FIG. 3, the physical storage is organized into a pluralityof distributed failure resilient address spaces (DFRASs) 518. Each ofwhich comprises a plurality of chunks 310, which in turn comprises aplurality of blocks 312. The organization of blocks 312 into chunks 310is only a convenience in some implementations and may not be done in allimplementations. Each block 312 stores committed data 316 (which maytake on various states, discussed below) and/or metadata 314 thatdescribes or references committed data 316.

The organization of the storage 308 into a plurality of DFRASs enableshigh performance parallel commits from many—perhaps all—of the nodes ofthe virtual filesystem (e.g., all nodes 104 ₁-104 _(N), 106 ₁-106 _(M),and 120 ₁-120 _(J) of FIG. 1 may perform concurrent commits inparallel). In an example implementation, each of the nodes of thevirtual filesystem may own a respective one or more of the plurality ofDFRAS and have exclusive read/commit access to the DFRASs that it owns.

Each bucket owns a DFRAS, and thus does not need to coordinate with anyother node when writing to it. Each bucket may build stripes across manydifferent chunks on many different SSDs, thus each bucket with its DFRAScan choose what “chunk stripe” to write to currently based on manyparameters, and there is no coordination required in order to do so oncethe chunks are allocated to that bucket. All buckets can effectivelywrite to all SSDs without any need to coordinate.

Each DFRAS being owned and accessible by only its owner bucket that runson a specific node allows each of the nodes of the VFS to control aportion of the storage 308 without having to coordinate with any othernodes (except during [re]assignment of the buckets holding the DFRASsduring initialization or after a node failure, for example, which may beperformed asynchronously to actual reads/commits to storage 308). Thus,in such an implementation, each node may read/commit to its buckets'DFRASs independently of what the other nodes are doing, with norequirement to reach any consensus when reading and committing tostorage 308. Furthermore, in the event of a failure of a particularnode, the fact the particular node owns a plurality of buckets permitsmore intelligent and efficient redistribution of its workload to othernodes (rather the whole workload having to be assigned to a single node,which may create a “hot spot”). In this regard, in some implementationsthe number of buckets may be large relative to the number of nodes inthe system such that any one bucket may be a relatively small load toplace on another node. This permits fine grained redistribution of theload of a failed node according to the capabilities and capacity of theother nodes (e.g., nodes with more capabilities and capacity may begiven a higher percentage of the failed nodes buckets).

To permit such operation, metadata may be maintained that maps eachbucket to its current owning node such that reads and commits to storage308 can be redirected to the appropriate node.

Load distribution is possible because the entire filesystem metadataspace (e.g., directory, file attributes, content range in the file,etc.) can be broken (e.g., chopped or sharded) into small, uniformpieces (e.g., “shards”). For example, a large system with 30 k serverscould chop the metadata space into 128 k or 256 k shards.

Each such metadata shard may be maintained in a “bucket.” Each VFS nodemay have responsibility over several buckets. When a bucket is servingmetadata shards on a given backend, the bucket is considered “active” orthe “leader” of that bucket. Typically, there are many more buckets thanVFS nodes. For example, a small system with 6 nodes could have 120buckets, and a larger system with 1,000 nodes could have 8 k buckets.

Each bucket may be active on a small set of nodes, typically 5 nodesthat that form a penta-group for that bucket. The cluster configurationkeeps all participating nodes up-to-date regarding the penta-groupassignment for each bucket.

Each penta-group monitors itself. For example, if the cluster has 10 kservers, and each server has 6 buckets, each server will only need totalk with 30 different servers to maintain the status of its buckets (6buckets will have 6 penta-groups, so 6*5=30). This is a much smallernumber than if a centralized entity had to monitor all nodes and keep acluster-wide state. The use of penta-groups allows performance to scalewith bigger clusters, as nodes do not perform more work when the clustersize increases. This could pose a disadvantage that in a “dumb” mode asmall cluster could actually generate more communication than there arephysical nodes, but this disadvantage is overcome by sending just asingle heartbeat between two servers with all the buckets they share (asthe cluster grows this will change to just one bucket, but if you have asmall 5 server cluster then it will just include all the buckets in allmessages and each server will just talk with the other 4). Thepenta-groups may decide (i.e., reach consensus) using an algorithm thatresembles the Raft consensus algorithm.

Each bucket may have a group of compute nodes that can run it. Forexample, five VFS nodes can run one bucket. However, only one of thenodes in the group is the controller/leader at any given moment.Further, no two buckets share the same group, for large enough clusters.If there are only 5 or 6 nodes in the cluster, most buckets may sharebackends. In a reasonably large cluster there many distinct node groups.For example, with 26 nodes, there are more than 64,000 (26!/5!*(26−5)!)possible five-node groups (i.e., penta-groups).

All nodes in a group know and agree (i.e., reach consensus) on whichnode is the actual active controller (i.e., leader) of that bucket. Anode accessing the bucket may remember (“cache”) the last node that wasthe leader for that bucket out of the (e.g., five) members of a group.If it accesses the bucket leader, the bucket leader performs therequested operation. If it accesses a node that is not the currentleader, that node indicates the leader to “redirect” the access. Ifthere is a timeout accessing the cached leader node, the contacting nodemay try a different node of the same penta-group. All the nodes in thecluster share common “configuration” of the cluster, which allows thenodes to know which server may run each bucket.

Each bucket may have a load/usage value that indicates how heavily thebucket is being used by applications running on the filesystem. Forexample, a server node with 11 lightly used buckets may receive anotherbucket of metadata to run before a server with 9 heavily used buckets,even though there will be an imbalance in the number of buckets used.Load value may be determined according to average response latencies,number of concurrently run operations, memory consumed or other metrics.

Redistribution may also occur even when a VFS node does not fail. If thesystem identifies that one node is busier than the others based on thetracked load metrics, the system can move (i.e., “fail over”) one of itsbuckets to another server that is less busy. However, before actuallyrelocating a bucket to a different host, load balancing may be achievedby diverting writes and reads. Since each write may end up on adifferent group of nodes, decided by the DFRAS, a node with a higherload may not be selected to be in a stripe to which data is beingwritten. The system may also opt to not serve reads from a highly loadednode. For example, a “degraded mode read” may be performed, wherein ablock in the highly loaded node is reconstructed from the other blocksof the same stripe. A degraded mode read is a read that is performed viathe rest of the nodes in the same stripe, and the data is reconstructedvia the failure protection. A degraded mode read may be performed whenthe read latency is too high, as the initiator of the read may assumethat that node is down. If the load is high enough to create higher readlatencies, the cluster may revert to reading that data from the othernodes and reconstructing the needed data using the degraded mode read.

Each bucket manages its own distributed erasure coding instance (i.e.,DFRAS 518) and does not need to cooperate with other buckets to performread or write operations. There are potentially thousands of concurrent,distributed erasure coding instances working concurrently, each for thedifferent bucket. This is an integral part of scaling performance, as iteffectively allows any large filesystem to be divided into independentpieces that do not need to be coordinated, thus providing highperformance regardless of the scale.

Each bucket handles all the filesystems operations that fall into itsshard. For example, the directory structure, file attributes and filedata ranges will fall into a particular bucket's jurisdiction.

An operation done from any frontend starts by finding out what bucketowns that operation. Then the backend leader, and the node, for thatbucket is determined. This determination may be performed by trying thelast-known leader. If the last-known leader is not the current leader,that node may know which node is the current leader. If the last-knownleader is not part of the bucket's penta-group anymore, that backendwill let the front end know that it should go back to the configurationto find a member of the bucket's penta-group. The distribution ofoperations allows complex operations to be handled by a plurality ofservers, rather than by a single computer in a standard system.

If the cluster of size is small (e.g., 5) and penta-groups are used,there will be buckets that share the same group. As the cluster sizegrows, buckets are redistributed such that no two groups are identical.

A failure domain is a set of storage devices that may fail (completelyor become temporarily unavailable) due to a single component failure. Ifthe failure of a single server will bring down a group of SSDs, thisgroup of SSDs on the single server may be considered a failure domain.If a rack has a single network switch, this rack could be considered afailure domain if the failure of the switch results in the whole rackbeing inaccessible. A failure domain may be configured uponinstallation. The configuration of a failure domain may also becontrolled from a Graphical User Interface (GUI), a Command LineInterface (CLI) or an Application Programming Interface (API). If nofailure domain definition is set, a single server may be used as thefailure domain. All of the SSDs present on that server may be treated asa large single SSD in terms of data placement.

FIG. 4 illustrates a representation of a distributed filesystem 400 inaccordance with an example implementation of this disclosure. Thedistributed filesystem 400 comprises a first failure domain 409 a, asecond failure domain 409 b and a third failure domain 409 c.

The first failure domain 409 a comprises at least one server 401 a andat least one storage device 411 a. The server 401 a comprises a firstfrontend 403 a and a first backend 405 a. The first backend 405 acomprises at least one bucket 407 a. The at least one storage device 411a may comprise a plurality of solid-state devices. The at least onestorage device 411 a may be configured into a plurality of blocks, e.g.,block a1 and block a2.

The second failure domain 409 b comprises at least one server 401 b andat least one storage device 411 b. The server 401 b comprises a secondfrontend 403 b and a second backend 405 b. The second backend 405 bcomprises at least one bucket 407 b. The at least one storage device 411b may comprise a plurality of solid-state devices. The at least onestorage device 411 b may be configured into a plurality of blocks, e.g.,block b1 and block b2.

The third failure domain 409 c comprises at least one server 401 c andat least one storage device 411 c. The server 401 c comprises a thirdfrontend 403 c and a third backend 405 c. The third backend 405 ccomprises at least one bucket 407 c. The at least one storage device 411c may comprise a plurality of solid-state devices. The at least onestorage device 411 c may be configured into a plurality of blocks, e.g.,block c1 and block c2.

The buckets 407 a, 407 b and 407 c are operable to build failureresilient stripes comprising a plurality of blocks. For example, bucket407 a in the first backend 405 a may build stripe 411, which comprisesblocks a1 and a2 of the first failure domain 409 a; blocks b1 and b2 ofsecond first failure domain 409 b; and blocks c1 and c2 of the thirdfailure domain 409 a. Two or more blocks of the plurality of blocks a1,a2, b1, b2, c1 and c2 are configured to comprise error correctioninformation.

Upon a failure of the first failure domain 409 a, blocks a1 and a2 maybe regenerated according to blocks b1, b2, c1 and/or c2. Upon a failureof the second failure domain 409 b, blocks b1 and b2 may be regeneratedaccording to blocks a1, a2, c1 and/or c2.

If the first failure domain 409 a and the second failure domain 409 blose communication with each other, the third failure domain 409 c isoperable to determine which of the first failure domain 409 a and thesecond failure domain 409 b will continue running the system. Neitherthe first failure domain 409 a nor the second failure domain 409 b willrebuild the failure resilient stripe 413 unless permission is granted bythe third failure domain 409 c. Blocks c1 and c2 of the third failuredomain 409 c may or may not comprise data that is used in rebuildingstripe 413.

A bucket 407 a in the first backend 405 a may be the initial leader ofthe failure resilient stripe 413. Upon the failure of the first failuredomain, however, a bucket 405 b of the second backend 405 b may becomethe leader of the rebuilt failure resilient stripe 413. The rebuiltfailure resilient stripe 413 may not use blocks a1 and a2 if the firstfailure domain 409 a is unavailable.

A large failure domain may be defined. For example, all of the SSDs onall the servers in that failure domain may be treated as if they wereone large SSD storage device when data placement and stripeconfiguration is considered. This allows the filesystem to withstand acomplete failure domain failure, as if there are no two data blocks fora same stripe on the same failure domain, it can always be rebuilt fromthe other failure domains.

Larger failure domains reduce the amount of stripe groups and increasethe rebuild time. Because the rebuild process may run from all availablecomputers in the other failure domains, all stripes may be rebuiltconcurrently.

The widest stripe size may be limited by the amount of failure domain.For example, with 10 failure domains, 8 blocks of data could beprotected with 2 blocks of error protection/correction (i.e., using an8+2 stripe). Likewise, with 10 failure domains, 6 blocks of data couldbe protected with 4 blocks of error protection/correction (i.e., using a6+4 stripe).

A failure domain may also limit the maximal amount of data placementsfrom a single stipe in each failure domain. For example, an organizationthat has 3 or 4 data centers in a metro area may run the cluster acrossall its data centers, so in case one data center fails, the remainingdata centers may continue to operate.

With 3 data centers, the filesystem can be protected with a 5+4 scheme,where no more than 3 pieces of data may be placed in the same failuredomain. In this example, if a data center fails, there are still twoothers data centers with at least 6 pieces of data from each stripe thatcould be used to rebuild. The case of 3 data centers can also use 4+2protection with no more than two pieces of data placed in the samefailure domain. With 4 data centers, for example, 4+2 protection can beused with no more than two pieces of data placed in the same failuredomain.

Two data centers may use 2+2 data protection with no more than two datapieces/blocks in each data centers. This case, however, requires thefilesystem to determine which data center has stayed alive, therebypreventing a “split brain” situation where the cluster just splits intotwo datacenters that start working independently. To prevent a “splitbrain” scenario, another instance/server may be added on a third datacenter. The third data center may monitor/control communication with thetwo data centers, and in case first and second data centers losecommunication with each other, that third data center (e.g., in a thirdfailure domain) may decide (and let the servers in first and second datacenters know) which half of the servers can continue running as asystem. Unless the tie breaker node (third data center) grants a halfcluster the permission to continue operating (and start rebuildingdata), it won't be promoted to work on its own.

When rebuilding during such situations, as long as the remaining failuredomain is all down, the filesystem will rebuild the data into theremaining failure domains that are up, to maintain high resiliency toindividual server failures. Once that failure domain goes back online,data will be redistributed (rebuilt again) to maintain the requitedmaximum amount of data pieces out of each stripe on each failure domain.

An availability group is a group of servers that failover together andbe considered a failure domain. While the availability group stays up,the availability group can access the data. When the availability groupgoes down together, other servers can still access the data over adefined data set of the system. Availability groups are a different wayof controlling the data distribution. An availability group chooses agroup of servers and a filesystem (e.g., a subset of a large namespace)that protect each other. An availability group may be in a specific roomin that data center, for example. As long as these servers are up, alldata for that filesystem will be stored in these servers, and not onother servers. So, even if other servers fail (e.g., other rooms of thedata center lost power) that filesystem will still be available on theseservers. An availability group may also be defines as all servers in asingle rack. As long as that rack is up and running, service cancontinue operate from that rack independent of the other racks of thecluster.

FIG. 5 is a flowchart illustrating an example method for generating adistributed filesystem in accordance with an example implementation ofthis disclosure. In block 501, a plurality of data pieces are receivedby a first failure domain. In block 503, a plurality of error correctionpieces are generated according to the plurality of data pieces. In block505, a failure resilient stripe comprising a plurality of blocks isbuilt by a first backend of the first failure domain. The failureresilient stripe may be built by a bucket in the first backend. Thisbucket in the first backend will be the leader of the failure resilientstripe until another bucket is promoted to leader.

Each block of the plurality of blocks comprises one data piece of theplurality of data pieces or one error correction piece of the pluralityof error correction pieces. In block 507, two or more blocks of thefailure resilient stripe are located in the first failure domain and twoor more other blocks of the failure resilient stripe are located in asecond failure domain.

In block 509 if the first failure domain fails, the blocks in the firstfailure domain are regenerated according to the blocks in the secondfailure domain. In block 511 if the second failure domain fails, theblocks in the second failure domain are regenerated according to theblocks in the first failure domain.

If there are only 2 failure domains in a network cluster, another devicemay detect that the first and second failure domains have lostcommunication with each other. This other device may then determinewhich of the first failure domain and the second failure domain willrebuild the failure resilient stripe. In certain embodiments, neitherthe first failure domain nor the second failure domain may rebuild thefailure resilient stripe unless permission is granted by a third failuredomain.

If the bucket leader is in a domain that fails, a bucket in anotherfailure domain may be promoted to become a leader of the failureresilient stripe when it is rebuilt.

While the present method and/or system has been described with referenceto certain implementations, it will be understood by those skilled inthe art that various changes may be made and equivalents may besubstituted without departing from the scope of the present methodand/or system. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the presentdisclosure without departing from its scope. Therefore, it is intendedthat the present method and/or system not be limited to the particularimplementations disclosed, but that the present method and/or systemwill include all implementations falling within the scope of theappended claims.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprisefirst “circuitry” when executing a first one or more lines of code andmay comprise second “circuitry” when executing a second one or morelines of code. As utilized herein, “and/or” means any one or more of theitems in the list joined by “and/or”. As an example, “x and/or y” meansany element of the three-element set {(x), (y), (x, y)}. In other words,“x and/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, circuitry is “operable” to perform a function wheneverthe circuitry comprises the necessary hardware and code (if any isnecessary) to perform the function, regardless of whether performance ofthe function is disabled or not enabled (e.g., by a user-configurablesetting, factory trim, etc.).

What is claimed is:
 1. A system comprising: a first storage device; asecond storage device; a first backend; a second backend; a firstfailure domain; and a second failure domain, wherein: the first backendand the second backend reach consensus that the first backend is anactive controller, if a data access is directed to the second backend,the second backend redirects the data access to the first backend, thefirst backend is operable to build a failure resilient stripe comprisinga first set of blocks located in the first storage device and a secondset of blocks located in the second storage device, upon a failure ofthe first failure domain, the first set of blocks is regeneratedaccording to the second set of blocks, and upon a failure of the secondfailure domain, the second set of blocks is regenerated according to thefirst set of blocks.
 2. The system of claim 1, wherein the first failuredomain comprises a plurality of solid-state drives.
 3. The system ofclaim 1, wherein the first failure domain comprises one or more servers.4. The system of claim 1, wherein the first failure domain comprises arack and a network switch.
 5. The system of claim 1, wherein if thefirst failure domain and the second failure domain lose communicationwith each other, a third failure domain is operable to determine whichof the first failure domain and the second failure domain will continuerunning the system by rebuilding data.
 6. The system of claim 1, whereinneither the first failure domain nor the second failure domain willrebuild the failure resilient stripe unless a permission is granted by athird failure domain.
 7. The system of claim 1, wherein the firstbackend comprises a bucket that initially builds the failure resilientstripe.
 8. The system of claim 1, wherein the first backend comprises abucket that is a leader of the failure resilient stripe.
 9. The systemof claim 1, wherein the second backend comprises a bucket that becomes aleader of the failure resilient stripe if the first failure domainfails, wherein the second failure domain comprises the second backend.10. The system of claim 1, wherein the first failure domain comprise anavailability group.
 11. A method comprising: reaching consensus, betweena first backend and a second backend, that the first backend is anactive controller; if a data access is directed to the second backend,redirecting the data access to the first backend; using the firstbackend to build a failure resilient stripe comprising a first set ofblocks located in a first storage device and a second set of blockslocated in a second storage device; upon a failure of a first failuredomain, regenerating the first set of blocks according to the second setof blocks; and upon a failure of a second failure domain, regeneratingthe second set of blocks according to the first set of blocks.
 12. Themethod of claim 11, wherein the first failure domain comprises aplurality of storage devices.
 13. The method of claim 11, wherein thefirst failure domain comprises one or more servers.
 14. The method ofclaim 11, wherein the first failure domain comprises a rack and anetwork switch.
 15. The method of claim 11, wherein the method comprisesdetermining which of the first failure domain and the second failuredomain will rebuild the failure resilient stripe if the first failuredomain and the second failure domain lose communication with each other.16. The method of claim 11, wherein neither the first failure domain northe second failure domain will rebuild the failure resilient stripeunless a permission is granted by a third failure domain.
 17. The methodof claim 11, wherein the method comprises building the failure resilientstripe via a bucket in the first backend.
 18. The method of claim 11,wherein a bucket of the first backend is a leader of the failureresilient stripe.
 19. The method of claim 11, wherein the methodcomprises promoting a bucket of the second backend to become a leader ofthe failure resilient stripe if the first failure domain fails, whereinthe second failure domain comprises the second backend.
 20. The methodof claim 11, wherein the first failure domain comprise an availabilitygroup.